The present invention relates to an electron gun for a cathode ray tube, and particularly to an electron gun for a color cathode ray tube, wherein the astigmatism is reduced and the convergence characteristic is enhanced.
Generally, as illustrated in FIG. 1, a cathode ray tube is formed such that a panel 21 having a shadow mask frame assembly 24 mounted in the inside thereof meets with a funnel 22 which holds an electron gun 1 in a neck 23 at the end of the funnel, and a deflection yoke 30 is installed on the external surface of the neck.
In the cathode ray tube 20, R (Red), G (Green), and B (Blue) electron beams emitted from electron gun 1 are optimally focused on the center of a phosphor layer formed on the inner surface of the panel 21. Also, even though the R, G, and B electron beams converge at one spot, when the three electron beams deflect toward the periphery of the phosphor layer, the trajectory of the beam is formed as illustrated in FIG. 1, so that the R, G, and B electron beams do not converge at one spot and, moreover, the beam spot becomes distorted due to astigmatism. These factors degrade color purity and resolution at the periphery of an image.
FIG. 2 is a schematic view illustrating a conventional electron gun for a cathode ray tube designed to solve above-described problem.
The electron gun illustrated in FIG. 2 is composed of a preceding triode consisting of cathodes 2, a control electrode 3, and a screen electrode 4, a main lens system having a static focus electrode 5 for focusing and accelerating electron beams, a dynamic focus electrode 6, and an anode 7. Vertically-elongated electron beam passing holes 5H are formed in the outgoing side 5a of static focus electrode 5 to correspond to horizontally-elongated electron beam passing holes 6H in the incoming side 6a of dynamic focus electrode 6. In this electron gun, static focus voltage Vsf and anode voltage Ve are respectively supplied to static focus electrode 5 and anode 7, and a parabolic dynamic focus voltage Vdf is supplied to dynamic focus electrode 6, which is synchronized with the vertical/horizontal synchronizing signals of the deflection yoke and its lowest voltage is the same as the static focus voltage.
In the conventional dynamic focus electron gun 1 formed as described above, when the electron beams emitted from cathodes 2 deflect toward the periphery of the phosphor layer due to the deflecting magnetic field of the deflection yoke, dynamic focus voltage Vdf (synchronized with vertical/horizontal deflection signals supplied to the deflection yoke) is supplied to dynamic focus electrode 6. Thus, a quadrupole lens can be formed between static focus electrode 5 and dynamic focus electrode 6, which compensates for the astigmatism of the electron beams deflecting toward the periphery of the image.
The specific description of this operation is presented as below. While the electron beams deflect toward the periphery of the phosphor layer, a dynamic focus voltage higher than the static focus voltage is applied to the dynamic focus electrode 6, so that a lens of weaker focusing force and stronger diverging force is formed in the vertical direction relative to the horizontal direction. Stated conversely, the lens has stronger focusing force and weaker diverging force in the horizontal direction relative to the vertical direction. Here, this lens is formed by the vertically-elongated electron beam passing holes 5H formed in the outgoing side 5a of static focus electrode 5, and the horizontally-elongated electron beam passing holes 6H formed in the incoming side 6a of dynamic focus electrode 6. Therefore, the electron beams passing through the lens are under the influence of a force which focuses in the horizontal direction and diverges in the vertical direction, so that the cross-sectional shape of the beams becomes vertically-elongated. When the deformed electron beam having a distorted cross-sectional shape deflects toward the periphery of the phosphor layer, the deflecting magnetic field of the deflection yoke 30 compensates the distortion of the electron beam caused by a non-uniform deflecting magnetic field. As a result, the same circular beam spot can be obtained at the periphery as at the center of the screen.
Also, since dynamic focus voltage Vdf whose lowest voltage is the same as the static focus voltage Vsf is supplied to dynamic focus electrode 6, the potential difference from anode 7 is relatively decreased, which weakens the intensity of the major lens formed between these two points and, in turn, the focusing distance of the electron beam is lengthened. Therefore, the electron beam is optimally focused at the periphery of the phosphor layer.
However, according to experiments of this applicant, for electron beams deflected toward the right and left sides of the phosphor layer, the electron beams land optimally when the potential difference between focus electrode 5 and dynamic focus electrode 6 is 900 V. When deflecting toward the phosphor layer's corners, the potential difference should be 1500 V.
The aforesaid conventional electron gun enables the focus characteristic to be enhanced and astigmatism to be improved. However, this electron gun has a limitation in that the convergence characteristic which makes the R, G, and B electron beams converge on one spot of the phosphor layer cannot be enhanced. As illustrated in FIG. 3, in order to overcome the functional restriction of the conventional electron gun, a cylindrical blade 5b' is fixed to the edge of the central electron beam passing hole among the electron beam passing holes formed in the outgoing side 5a' of a focus electrode 5', and semi-circular blades 5c' toward the central electron beam passing hole are each fixed to the inner edges of the flanking electron beam passing holes. Also, semi-circular blades 6b' are fixed to the outer edges of the flanking electron beam passing holes, and are formed in the incoming side 6a' of a dynamic focus electrode 6'. Therefore, the convergence is enhanced throughout the entire phosphor layer. However, in this case, even though dynamic focus voltage Vdf is supplied to dynamic focus electrode 6', since a quadrupole lens for compensating astigmatism is not formed, the distortion of the electron beam due to a non-uniform magnetic field of the deflection yoke 30 at the periphery of the phosphor layer cannot be corrected when the electron beam deflects. For this reason, the image resolution of the cathode ray tube deteriorates.